Vehicle suspensions

ABSTRACT

An active vehicle roll control system is disclosed in which a roll bar ( 22 ) has two halves ( 22   a,    22   b ) which can be rotated relative to each other by a control unit ( 26 ) in order to actively control vehicle roll. The control unit ( 26 ) receives signals from lateral accelerometers ( 29, 30 ). In order to determine accurately when the vehicle is experiencing zero lateral acceleration, the control unit ( 26 ) averages the signal from each accelerometer ( 29, 30 ) continuously and uses the average signal as an indication of the straight ahead position. The averaging algorithm defines a window of low lateral accelerations to be used, and a gain to be applied to the acceleration signals, each of which is speed dependent so that the averaging process is not affected by abnormal vehicle movements.

The present invention relates to accelerometers used in activesuspension systems for vehicles, and in particular to their calibration.

BACKGROUND OF THE INVENTION

It is known, for example as discussed in U.S. Pat. No. 5,742,919, thataccelerometers used for controlling vehicle suspensions are subject todrift and other sources of inaccuracy, and that they need to becalibrated or checked in some way. The method proposed for this in U.S.Pat. No. 5,742,919 is to use different sensors on the vehicle to measurethe same parameter, and thereby check that the measured values arecorrect. However, this requires there to be a large number of sensorsand a certain amount of duplication of measurements.

SUMMARY OF THE INVENTION

The present invention provides a vehicle suspension system including acontrol means arranged to respond to signals from a lateralaccelerometer arranged to measure lateral acceleration of the vehiclecharacterized in that the control means is arranged to measure anaverage of the signal from the accelerometer over time to determine areference signal which corresponds to zero lateral acceleration, and tocompare the instantaneous signal with the reference signal to measurethe instantaneous lateral acceleration.

Preferably the control means is arranged to continually update theaverage during use of the vehicle. This ensures that any drift in theaccelerometer is compensated for.

For example the average may be produced by integrating over time thevalue of the signal from the accelerometer.

Preferably the average is a weighted average. For example weightingfactor may be used which preferably dependent on the instantaneous speedof the vehicle is arranged to bias the averaging towards signalsproduced when the lateral acceleration is low. Preferably the weightingfactor is greatest at a predetermined vehicle speed and falls off atspeeds higher than said predetermined speed and at speeds lower thansaid predetermined speed.

Preferably signals corresponding to lateral accelerations above apredetermined maximum value, which may vary with the speed of thevehicle, and is preferably zero for low vehicle speeds, are not includedin the averaging process.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings inwhich:

FIG. 1 is a diagrammatic representation of a vehicle including asuspension according to an embodiment of the invention,

FIGS. 2 and 3 are graphs showing zero learning characteristics of anaccelerometer forming part of the system of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a four-wheel-drive off-road vehicle has four wheels10, 12, 14, 16 each mounted on the vehicle body 18. The vehicle has anindependent suspension, each of the wheels being attached to the body 18through a suspension arm 20 so that it can move vertically relative tothe body 18. An anti-roll bar 22 is connected between the two rearwheels 14, 16 to control the roll of the rear of the vehicle. Theanti-roll bar 22 is split in the middle into two halves 22 a, 22 b whichcan be rotated relative to each other by a rotary actuator 24 under thecontrol of a control unit 26. This enables vehicle roll to be controlledactively in response to signals input to the control unit from wheelspeed sensors 27 first and second lateral accelerometers 29, 30 whichprovide signals indicative of the acceleration of parts of the vehiclebody in various directions. A similar anti-roll bar, which is not shown,would also normally be connected between the front wheels 10, 12. Thecontrol unit 26 can control the actuator 24 so as to tend to resistvehicle roll by applying a torque between the two halves of theanti-roll bar, the magnitude of which is dependent on the lateralacceleration of the vehicle as measured by the accelerometers 29, 30.

Clearly in order to operate accurately the control unit needs to be ableto determine accurately the lateral acceleration of the vehicle from thesignals it receives from the accelerometers 29, 30. However, thesesignals tend to drift over time and to change with temperature.Accordingly the control unit is arranged to continuously monitor theaccelerometer signals over time and to determine from them a referencesignal for each accelerometer corresponding to zero lateralacceleration. This reference signal is continually updated during use ofthe vehicle. Then to measure lateral acceleration it can compare theinstantaneous signal from each accelerometer with the reference signalto measure the instantaneous lateral acceleration experienced by theaccelerometer.

The reference signal is determined, or learnt, by means of calculating arunning average of the accelerometer signal over time. This is done bycontinuously integrating the accelerometer signal. However the averageincludes a weighting factor, in the form of a signal gain on the inputto the integration, which varies with road speed as shown in FIG. 2. Thegain is zero at road speeds below 10 mph from where it increases towardsa peak at 50 mph and then falls off again at higher speeds. This gaincharacteristic is arranged so that the averaging process is biasedtowards speeds when the vehicle is likely to be travelling in a straightline and lateral accelerations are likely not to be high. It is zero atvery low speeds where the vehicle is likely to be off road andexperiencing high lateral accelerations, and is low at high speeds wheneven gentle cornering can produce high lateral accelerations.

Referring to FIG. 3, the zero learning algorithm also defines a windowof low lateral accelerations which are taken into account in theaveraging process, i.e. which are included in the integrating process.Any larger lateral accelerations, the magnitude of which is outside thewindow, are ignored and therefore not included in the integration. Ascan be seen, the breadth of the window is dependent on the ground speedof the vehicle. At low speeds, below 10 mph, the window has zero width.This is consistent with the zero gain at such speeds and ensures that,at low speeds, all accelerations are ignored. This is so that if thevehicle is parked or travelling slowly on a side slope, this will notaffect the centring effect of the averaging process. From 10 mph thewindow broadens out rapidly to a maximum width of 0.15 g at 14 mph. Itthen drops off at a gradually decreasing rate as the vehicle speedincreases up to 100 mph, where the width of the window is 0.015 g. Thereason for this is that, as with the gain control, the algorithm isintended to learn mostly, or exclusively, on normal, straight ahead,driving. At lower speeds rough roads and sharp bends can producerelatively high lateral accelerations during such driving, hence thewide window at lower speeds. At higher speeds roads will generally besmoother and corners more gentle, so any lateral acceleration above alow limit, of the order of 0.02 g at 80 mph for example, is indicativeof abnormal driving conditions which do not want to be included in theaveraging process.

It will also be understood that the present invention is suitable forroll control systems using any of a variety of known actuators tocontrol vehicle roll. For example independent air suspension systemswhich include a roll control strategy in the control of air pressure inthe gas struts at each of the wheels could use the present invention, ascould systems using an anti-roll bar with other forms of actuator, suchas those having a hydraulically operated strut to control rotation ofone end of the anti-roll bar about the central torsion part of the bar,as shown, for example, in WO98/26948.

What is claimed is:
 1. A vehicle suspension system including a lateralaccelerometer arranged to produce a lateral acceleration signal, and acontrol means arranged to receive the lateral acceleration signal, tomeasure an average value of the signal over time and to determine fromthe average value a reference value which corresponds to zero lateralacceleration, and to measure an instantaneous value of the signal andcompare the instantaneous value with the reference value to measure aninstantaneous lateral acceleration, the average value is a weightedaverage produced using a weighting factor and by integrating over timethe instantaneous value of the lateral acceleration signal.
 2. Thesystem according to claim 1, wherein the weighting factor is arranged tobias the average value towards the value of signals produced when thelateral acceleration is low.
 3. The system according to claim 2, whereinthe control means is arranged to define a maximum value of the lateralacceleration and, when determining the average value, to exclude signalscorresponding to lateral accelerations above the maximum value.
 4. Thesystem according to claim 3, further including a vehicle speed sensorwherein the control means is arranged to monitor vehicle speed using thespeed sensor and to vary the maximum value in response to variations inthe vehicle speed.
 5. The system according to claim 4, wherein thecontrol means is arranged to define said maximum value as zero when thevehicle speed is below a predetermined minimum speed.
 6. The systemaccording to claim 1, further comprising a vehicle speed sensor formeasuring an instantaneous speed of the vehicle, wherein the controlmeans is arranged such that the weighting factor is dependent on theinstantaneous speed of the vehicle.
 7. The system according to claim 6,wherein the control means defines a predetermined vehicle speed, and isarranged such that the weighting factor is greatest at the predeterminedvehicle speed and falls off at speeds higher than said predeterminedvehicle speed and at speeds lower than said predetermined vehicle speed.8. The system according to claim 7, wherein said predetermined vehiclespeed is about 50 mph.
 9. A vehicle suspension system including alateral accelerometer arranged to produce a lateral acceleration signal,and a control means arranged to receive the lateral acceleration signal,to measure an average value of the signal over time and to determinefrom the average value a reference value which corresponds to zerolateral acceleration, and to measure an instantaneous value of thesignal and compare the instantaneous value with the reference value tomeasure an instantaneous lateral acceleration; and wherein the averagevalue is produced by integrating over time the instantaneous value ofthe lateral acceleration signal and the average value is a weightedaverage produced using a weighting factor which is arranged to bias theaverage value towards the value of signals produced when the lateralacceleration is low.
 10. The system according to claim 9, wherein thecontrol means is arranged to define a maximum value of the lateralacceleration and, when determining the average value, to exclude signalscorresponding to lateral accelerations above the maximum value.
 11. Thesystem according to claim 10, further including a vehicle speed sensorwherein the control means is arranged to monitor vehicle speed using thespeed sensor and to vary the maximum value in response to variations inthe vehicle speed.
 12. The system according to claim 11, wherein thecontrol means is arranged to define said maximum value as zero when thevehicle speed is below a predetermined minimum speed.
 13. The systemaccording to claim 9, further comprising a vehicle speed sensor formeasuring an instantaneous speed of the vehicle, wherein the controlmeans is arranged such that the weighting factor is dependent on theinstantaneous speed of the vehicle.
 14. The system according to claim13, wherein the control means defines a predetermined vehicle speed, andis arranged such that the weighting factor is greatest at thepredetermined vehicle speed and falls off at speeds higher than saidpredetermined vehicle speed and at speeds lower than said predeterminedvehicle speed.
 15. The system according to claim 14, wherein saidpredetermined vehicle speed is about 50 mph.